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The term eggshell is a term for the outer
covering of a hard-shelled egg, and
some forms of eggs with soft outer coats.

The generalized eggshell structure, which varies
widely among species, is a protein matrix lined with
mineral crystals, usually of a calcium compound such as
calcium
carbonate. It is calcium build-up and is not made of cells.
Harder eggs are more mineralized than softer eggs.

Diversity

Insect eggs

Insects and other
arthropods lay a
variety of styles and shapes of eggs. Some have gelatinous or
skin-like coverings, others have hard eggshells. Softer shells are
mostly protein. It may be fibrous or quite liquid. Some arthropod
eggs do not actually have shells, rather, their outer covering is
actually the outermost extraembryonic membrane, the chorion, which
serves to protect inner layers. The chorion itself can be a complex
structure, and it may have different layers within it. It may have
an outermost layer called an exochorion. Eggs which must survive in
dry conditions usually have hard eggshells, made mostly of
dehydrated or mineralized proteins with pore systems to allow
respiration.
Arthropod eggs can have extensive ornamentation on their outer
surfaces.

Fish, amphibian and reptile eggs

Fish and amphibians generally lay eggs
which are surrounded by the extraembryonic membranes but do not
develop a shell, hard or soft, around these membranes. Some fish
and amphibian eggs have thick, leathery coats, especially if they
must withstand physical force or desiccation.

While many reptiles lay eggs with flexible,
calcified, eggshells, there are some that lay hard eggs. Eggs laid
by snakes generally have leathery shells which often adhere to one
another. Depending on the species, turtles and tortoises lay hard
or soft eggs. Several species lay eggs which are nearly
indistinguishable from bird eggs. Hatching reptiles, like most
birds, have egg-teeth with
which they slit or peck an opening into the shell to help them
emerge.

Bird eggs

Birds are known for
their hard-shelled eggs. The eggshell is approximately 95% calcium
carbonate crystals, which are stabilized by a protein matrix.
Without the protein, the crystal structure would be too brittle to
keep its form. The standard bird eggshell is a semipermeable
membrane filled with thousands of pores and covered on its
outer surface with a cuticle (called the bloom if it
is around a chicken egg), which helps the egg retain its water and
keep out bacteria. As
an egg develops within a bird's reproductive system, the shell is
laid down as a liquid mineral layer around the already-formed
extraembryonic membranes. This takes place in a part of the bird's
oviduct which is called
the uterus and serves a very different purpose from the uterus of a
mammal. The shell is laid
down by shell glands. In an average laying hen, the process of shell
formation takes around 20 hours. Pigmentation is
added to the shell, coloring it any of a variety of colors and
patterns depending on species. The waxy cuticle is deposited on the
shell in the vagina of the bird's oviduct. The chick has an
egg-tooth
which it uses to start a hole in the hard eggshell to allow it to
hatch.

Mammal eggs

Monotremes,
egg-laying mammals, lay soft-shelled eggs similar to those of
reptiles. The shell is deposited on the egg in layers within the
uterus. The egg can take up fluids and grow in size during this
process, and the final, most rigid layer is not added until the egg
is full-size.